Transistors in which an active layer is formed using a semiconductor thin film which is formed over a substrate having an insulating surface are widely used in electronic devices such as integrated circuits (ICs) and image display devices (also simply referred to as display devices).
In general, formation of a highly-integrated circuit requires miniaturization of a transistor. Conventional thin film transistors mostly have what is called a planar structure, in which a semiconductor film, an insulating film, and an electrode are stacked over a plane. As a transistor for a highly-integrated semiconductor device, a fin-type transistor in which a polycrystalline silicon film is used as an active layer is disclosed.